The present invention relates to a hydrometallurgical process for extracting noble metals from raw materials containing the same.
The literature is replete with descriptions of various processes for recovering noble metals, and especially gold and silver, from raw materials containing the same.
Thus, e.g. a method is known whereby gold and other noble metals are extracted by treating concentrates and ores as well as secondary wastes of jewellery and other industries with solutions of cyanides of alkali metals with subsequent separation of noble metals with the aid of zinc powder, by solution regeneration and refining of separated noble metals.
This type of method is described in British Patent No. 2,156,794 in which carbonaceous Au ores are leached with an alkali cyanide solution containing .gtoreq.0.3% alkali hydroxide for .gtoreq.5 kg/ton ore in excess of neutralization requirements.
There also exists a method of extracting gold and other noble metals by leaching in thiourea solutions with subsequent precipitation (cementation) by zinc powder, regeneration of thiourea and zinc, and refining of separated noble metals. Thus, as described by Won, Chang Whan; Cho, Tong Rae (Coll. Eng., Chungnam Univ., S. Korea). Taehan Kwangsan Hakhoe Chi. 1985, 22(3), 215-230 (Korean), the dissolution of Au in acidic solutions of thiourea containing Fe.sup.3+ oxidant was studied. By using a rotating pure disk, Au can be dissolved at much greater rate by acidic solutions of thiourea than is possible by conventional cyanidation. Dissolution rates are dependent on thiourea and oxidant concentration and are controlled partially by chemical phenomena and partially by transport of ions (thiourea and oxidant) to the surface.
There exist also methods of extracting gold and other noble metals by dissolving raw materials in acid solutions in the presence of strong oxidants, e.g. aqua regia, elementary chlorine, etc.
In U.S. Pat. No. 4,551,213 Au is recovered from the slurries from hydrometallurgical processing of Cu sulfide ores by: (a) adding .gtoreq.12% chloride; (b) maintaining oxidation potential of the slurry at 650-750 mV between room temperature and .+-.106.degree., optionally with addition of a strong oxidizer such as Cl gas; and (c) adding CuCl.sub.2 or FeCl.sub.3 to promote ionization of Au to form AuCl.sub.4.sup.-. The loaded solution is treated for Au recovery by electrolysis or adsorption on powdered C. The spent electrolyte is reoxidized in an anode compartment, and recycled for pulp treatment in a continuous process. The Au is optionally recovered from wastes and scrap in manufacturing of electrical circuit boards.
Similarly in Indian Patent No. 155,454 there is described a process wherein lean ores containing Ag and optionally Au are leached in concentrated chloride solutions or brines to promote formation of AgCl and to enhance solubility by complexing with the chloride. Recovery of Au is improved by addition of a strong oxidant such as Cl gas or a hypochlorite, as well as pH control in leaching.
Existing processes involve the use of extremely harmful toxic substances (e.g. cyanides, nitric oxides, elementary chlorine etc.) which require the creation of very expensive special protective measures for the personnel and environment. Moreover, the existing processes do not allow poor raw materials to be processed with sufficient economic efficiency, and involve many complicated stages and high labour content. Some raw materials, such as clay ores and graphite crucible wastes used for gold melting, cannot be processed at all by existing methods.